This invention pertains to non-aqueous rechargeable lithium manganese oxide batteries with greatly improved cycling performance at elevated temperatures, and methods of producing such batteries. Specifically, the invention pertains to using deposits of certain metal compounds, particularly ones containing Y, Bi, Pb and La, on the surface of a spinel cathode as means to stabilize the spinel surface, thereby avoiding the capacity loss.
Various types of non-aqueous rechargeable lithium ion batteries are available commercially for consumer electronics applications. Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. Presently available lithium ion batteries are high voltage systems based on LiCoO2 cathode and coke or graphite anode electrochemistries. However, many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO2 and LiMn2O4. Also, a wide range of carbonaceous compounds is suitable for use as the anode material. These batteries employ non-aqueous electrolytes comprising LiBF4 or LiPF6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
The rechargeable lithium battery industry has found that LiMn2O4 can be a more desirable cathode material than LiCoO2, because of its low cost and its relative harmless effect on the environment. Therefore, research efforts to use LiMn2O4 as the cathode material of choice have increased.
Typically LiMn2O4 based batteries have good performance at room temperature. However, at elevated temperatures they suffer a gradual loss of delivered capacity with cycle number, herein referred to as capacity fade or the capacity fade rate. Researchers in the art have devoted substantial effort to reducing this loss in capacity.
There are many patents/patent applications and articles in the literature claiming that doping with a foreign metal or a combination of metals during the synthesis of LiCoO2 or LiMn2O4 improves capacity and/or capacity fade. For instance, U.S. Pat. No. 5,147,738 (Yoshinori Toyoguchi) claims improved cycle life of LiCoO2 batteries by using cathode active material containing LixCo1xe2x88x92yMyO2, where Mxe2x95x90W, Mn, Ta, Ti, Nb; Japanese published application serial number 09134723 (Okada et. al.) uses LiyMn2xe2x88x92xMxO4, cathodes where Mxe2x95x90Fe, Ti, Ni, Ta, Cr, W, Pb, etc. to obtain large total discharge capacity; U.S. Pat. No.5,759,720 (Glenn Amatucci) discloses capacity and capacity fade improvement at 55xc2x0 C. for lithium rechargeable batteries using lithium aluminum manganese oxy-fluoride cathodes.
Addition of Bi is known to improve the stability of manganese oxides used in the so called RAM (Rechargeable Alkaline Manganese) battery technology. In the context of such aqueous alkaline cells, D. Larcher et. al. (J. Electrochem. Soc., Vol. 145, No.10, pp.3392-3400) study the effects of Bi, Pb and Tl doping, on the stability of xcex-MnO2 (the de-lithiated form of LiMn2O4) when stored in aqueous acidic media. Larcher et al. characterize stability by measuring the rate at which Mn is dissolved from the solid phase, and no capacity fade measurements where carried out in electrochemical cells. Brief mention is made of the storage characteristics of Bi doped LiMn2O4 in non-aqueous acidic electrolyte, and it appears from the reported X-ray data that the stability was not improved by Bi doping.
Matsushita Electric Co. Ltd.""s laid open Japanese application serial number 05047384 (Yamaura et. al.). claims improved overdischarge at high temperature by using their cathode. The cathodes here are made from powder obtained from simply mixing spinel powder with metal oxide powder. The mixed powders are not heat treated before they are made into a slurry for cathode coating. This is definitely not a surface coating technique.
Coating the surface of LiMn2O4 to obtain specific effects has been investigated. For example, in U.S. Pat. No.5,705,291 (Amatucci et al.) the self discharge rate of batteries stored at 55xc2x0 C. is reduced by treating the surface of the lithium intercalating cathode material with a passivating layer containing an annealed coating composition of boron oxide. The inventors show no evidence that capacity fade is improved by this treatment.
Thus far, the compounds and methods attempted in the prior art either give marginal improvements in capacity fade rate at elevated temperatures or attempt to solve other problems facing lithium manganese oxide batteries.
Rechargeable batteries exhibit a loss in delivered capacity as a function of the number of charge/discharge cycles. Herein, the fractional loss of capacity per cycle is referred to as the capacity fade rate. The instant invention includes non-aqueous rechargeable lithium manganese oxide batteries having greatly improved capacity fade rates at elevated temperatures and methods for achieving the improved capacity fade rate. Non-aqueous rechargeable spinel batteries generally comprise a lithium manganese oxide cathode, a lithium compound anode, a separator and a non-aqueous electrolyte comprising a lithium salt dissolved in a non-aqueous solvent and are hereinafter called spinel or Li1+xMn2xe2x88x92xO4 (0 less than =x less than 0.33) batteries. We have discovered that heating a mixture of a small amount of one or more of certain foreign metal compounds with Li1+xMn2xe2x88x92xO4 powder can result in a reduced capacity fade rate at elevated temperatures in spinel lithium ion batteries. Preferably, the temperature applied to heat the mixture is high enough so that the foreign metal compound is converted to foreign metal products, which cover the surface of the spinel and do not enter the bulk of the spinel structure. Such treated spinel powders serve to reduce the capacity fade of spinel lithium-ion batteries at elevated temperatures. Hereinafter synthesized Li1+xMn2xe2x88x92xO4 powder will be referred to as ready-made spinel. This is to distinguish it from either the actual synthesis of spinel, where precursors such as EMD-MnO2 and Li2CO3 are heated to obtain Li1+xMn2xe2x88x92xO4 or from doped spinel, where for example, EMD-MnO2, Li2CO3 and Bi2O3 are heated to obtain Li1+xMn2xe2x88x92xxe2x88x92yBiyO4.
During cycling, lithium-ion cells tend to produce alcohols such as methanol or ethanol as a result of reactions between trace amounts of H2O and the materials forming the Solid Electrolyte Interface (SEI) layer on the anode (Aurbach et. al., J. Electrochem. Soc. 141, L1 (1994), ibid. 142, 1746 (1995), ibid. 142, 2873 (1995)). The alcohol in turn oxidizes on the surface of the charged or de-lithiated cathode, and produces more water. Water then reacts irreversibly with more SEI material and possibly with some intercalated lithium in the anode. Once intercalated Li has reacted with water to form LiOH, it is permanently inactive and the cell capacity decreases correspondingly. In particular, the effect is most prominent in spinel lithium ion batteries at elevated temperature as shown in Yu Wang et al., co-inventors of this invention (Poster III, The 9th International Meeting on Lithium Batteries, Edinburgh, Scotland, July 1998).
The rate of methanol reaction can be used to gauge whether specific treatments of ready-made spinel powder will improve the capacity fade of spinet batteries at elevated temperatures. That is, the charged or de-lithiated spinel, henceforth known herein as xcex-MnO2, in question can be soaked in a known amount of methanol, and then the water produced from the oxidation reaction of methanol and the cathode can be measured over time. The less water produced by a surface treated cathode relative to a reference cathode of untreated spinel, the lower the capacity fade rate is expected when the lithium-ion battery is cycled at elevated temperatures.
We have found that not all foreign metal surface treatments work to improve the capacity fade rate of spinel batteries. So far, according to our research, the foreign metal compounds that are able to achieve a significant capacity fade improvement contain either bismuth (Bi), lead (Pb), lanthanum (La), barium (Ba), zirconium (Zr), yttrium (Y), strontium (Sr), zinc (Zn) or magnesium (Mg). In particular, lead acetate, lead basic carbonate, lead stearate, lead nitrate, bismuth nitrate, bismuth hydroxide, bismuth oxycarbonate, bismuth acetate, bismuth oxide, lanthanum carbonate, lanthanum perchlorate, lanthanum nitrate, barium nitrate, zinc acetate, and zirconium di-nitrate oxide,yttrium (III) nitrate, strontium nitrate, zinc acetate and magnesium nitrate can be used to reduce the fade rate at elevated temperatures.
One of the preferred methods for obtaining the surface treated cathode powder is to dissolve. a suitable amount of one or more of certain foreign metal compounds into water and then mix it with ready-made spinel powder for less than an hour. After drying at about 95xc2x0 C. for an hour, the resulting mixture is further heated at above the decomposition temperature of the foreign metal compound, and less than or equal to 750xc2x0 C., for 2 to 4 hours. Preferably the heating temperature is 350xc2x0 C. Heating removes the moisture and forms the foreign metal product on the surface of the spinel. This method will henceforth be called aqueous treatment or aqueous treated.
To make it easier for processing large quantities of treated spinel powder, another preferred method is to dry mix the foreign metal compound with the ready-made spinel, then heat the mixture initially at above the decomposition of the foreign metal compound for about 1 hour, then further heat it at about 600xc2x0 C. and up to 750xc2x0 C. for 1.5 hours. Hereinafter this method is called dry-mix treatment. The prepared powders are then ready for cathode making and subsequently for battery assembling.
Herein the amount of foreign metals used for the surface treatment will be expressed in terms of the number of foreign metal atoms per spinel formula unit and will be referred to as mole % foreign metal:       %    ⁢          xe2x80x83        ⁢    M    =                    N        M                    N                              LI                          1                              +                2                                              ⁢                      Mn                          2                              -                2                                              ⁢                      O            4                                xc3x97    100    ⁢    %  
wherein M is the foreign metal and NM is the number of the foreign metal atoms compared to the number of spinel formula units. It has been discovered that adding an amount of one or more certain foreign metal compounds to the ready-made spinel in the range from 0.01 to less than about 5 mole % of the foreign metal, then heating the mixture, can be effective in improving capacity fade rate at elevated temperature. A preferred range is from about 0.05% to less than 2 mole % of the foreign metal. Preferably only a small amount of capacity fade reducing foreign metal compound is employed.
It has further been discovered that the inventive treatments result in the foreign metal products being distributed on the surface of the spinel structure rather than being embedded in the spinel structure, as in the case of doping. That is, the foreign metal species do not enter the bulk of the spinel particles or the crystal structure. Therefore, the spinel structure and its lattice constant remain essentially unchanged.
Aside from treating the spinel material, improved capacity fade rates at elevated temperatures can be achieved for spinel batteries by employing conventional lithium ion battery electrochemistries. Thus, the cathode comprises a surface treated spinel made by heating a mixture of one or more certain foreign metal compounds (bismuth, lead, lanthanum, barium, zirconium, yttrium, strontium, zinc or magnesium compounds) with ready-made lithium manganese oxide. The anode can be a carbonaceous insertion compound anode, in particular graphite. The electrolyte can contain LiPF6 salt dissolved in an organic carbonate solvent, in particular mixtures containing ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and/or diethyl carbonate solvents.
The invention is directed to a non-aqueous rechargeable lithium manganese oxide battery having reduced capacity fade rate during cycling at elevated temperature, the said battery comprising: a lithium insertion compound cathode, a lithium compound anode, a separator, and a non-aqueous electrolyte including a lithium salt dissolved in a non-aqueous solvent, wherein the cathode comprises a spinel structure composed of a mildly heated mixture of a ready-made spinel and one or more foreign metal compounds, the foreign metals having atomic number greater than 11; the foreign metal products being distributed on the surface of the spinel structure and not substantially entering the bulk of the spinel structure or substantially extracting lithium from the spinel structure, the surface distributed foreign metal compound reducing capacity fade rate of the battery during cycling at elevated temperature.
We find that the reduced capacity fade rate at elevated temperature is achieved when the ppm amount of H2O generated per gram of any charged-surface-treated spinel, k, is xe2x80x9csubstantially less thanxe2x80x9d the ppm amount of H2O generated per gram of charged-untreated spinel, ko. Here ppm of H2O generated in the methanol reaction is defined as H2O generated (xcexcg)/methanol (g), which must then be further normalized by the quantity of charged spinel. For our tests we used 1.4 g of methanol per gram of charged spinel. In this case we can quantify the meaning of xe2x80x9csubstantially less thanxe2x80x9d by looking at the statistical variance in our data. We have chosen 40 hours at 45xc2x0 C. as the standard storage condition for determining k, and find that the standard deviation in ko is about 0.1ko (i.e. 10%). Hence, we expect a substantially reduced capacity fade rate at elevated temperature when k less than 0.9ko.
The invention is also directed to a method for reducing the capacity fade rate during cycling at elevated temperature of a non-aqueous rechargeable lithium manganese oxide battery, the battery comprising a lithium insertion compound cathode, a lithium compound anode, a separator, a non-aqueous electrolyte including a lithium salt dissolved in a non-aqueous solvent, wherein the lithium insertion compound cathode is formed of a mildly heated mixture of a ready-made spinel and one or more foreign metal compounds, the foreign metals having atomic number greater than 11; the foreign metal products being distributed on the surface of the spinel structure and not entering the bulk of the spinel structure.
At this time, the reason for the capacity fade rate improvement is unclear. Without being adversely bound by theory, but wishing to enable the reader to better understand the invention, a possible explanation is that during moderate temperature heating of ready-made spinel with the foreign metal compounds, foreign metal products form and deposit on the surface of spinel to stabilize the spinel surface. The inventors believe that these foreign metal products are in the form of oxides for the examples presented herein. It is also believed that foreign metal products other than oxides, which also achieve the desired effect may also exist. For the examples that follow, the foreign metal products are hereafter called foreign metal oxides. Moderate heating is necessary to avoid diffusion of foreign metal atoms into the bulk of the spinel, thereby avoiding capacity loss. Therefore, the benefits of the invention might be expected when using certain foreign metal compounds which decompose at moderate temperatures to form foreign metal products on the surface of spinel.